The Precambrian

Evidence from the oldest rocks

These are the oldest rocks known regionally. They are part of the ancient cratonic backbone of the North American continent. Rocks such as these are widespread between Lake Mead and the eastern parts of the continent, but in most regions they are buried deep beneath younger layered rocks where they form the subsurface basement of much of the US. They lie buried deep beneath the Colorado Plateau to the east of Lake Mead but they are exposed at the bottom of the Grand Canyon, because of the deep incision made by the down-cutting Colorado River. They are exposed in several places around Lake Mead because of profound local tectonic uplifts, above which all the younger rocks have been removed. Great heat and pressure, both of which are products of deep burial and tectonic forces, profoundly changed these rocks from their original sedimentary, volcanic, and plutonic states. The change, called metamorphism, involved intense re-crystallization and resulted primarily in the formation of gneiss and schist. In most cases it is not possible to tell how long ago the original strata were deposited nor when much of the primary volcanism or plutonism took place. Radiometric dating has shown that most of these rocks cooled following the metamorphic event about 1.7 billion years ago and that some large plutons were intruded about 1.4 billion years ago. The largest of these plutons occurs at Gold Butte which is due east of the Virgin Basin.

Note the lobate form of the lighter material. As the rock heats and melts, the crystaline mash flows and forms pods of molten rock, which recystallizes with a different appearance from the parent rock.

Most of the metamorphic crystalline rocks have not been given formal names. One exception is the large plutonic mass at Gold Butte, which is called the Gold Butte Granite.

Proterozoic crystalline basement rocks can be found at the west base of Frenchman Mountain, on Saddle Island, at a few localities along Wilson Ridge and in Boulder Canyon, in a large region around southern Iceberg Canyon and Gold Butte, the south Virgin Mountains, Black Ridge, and on the north/northeast side of Virgin Peak.